Field of the Invention
This invention relates generally to the field of semiconductor device fabrication and, more particularly, to the deposition of titanium nanolaminate films. The titanium nanolaminates can be used, for example, in processes for forming integrated circuits.
Description of the Related Art
There is an extremely high demand for integrated circuits to be decreased in size. This demand stems, for example, from a need for increased portability, increased computing power, increased memory capacity, and increased energy efficiency. In order to decrease the size of integrated circuits, the sizes of the constituent features, electrical devices and interconnect lines, for example, must be reduced as well.
The demand for reduced size has moved the industry to continuously reduce constituent feature size in integrated circuits. For example, transistors or memory circuits or devices such as dynamic random access memories (DRAMs), flash memory, static random access memories (SRAMs), ferroelectric (FE) memories, field effect transistors (FETs), such as multiple gate FETs, like FinFET's, are continuously being made smaller.
One example, DRAM typically comprises millions of identical circuit elements, known as memory cells. In its most general form, a memory cell typically consists of two electrical devices: a storage capacitor and an access field effect transistor. Each memory cell is an addressable location that can store one bit (binary digit) of data. A bit can be written to a cell through the transistor and can be read by sensing charge in the capacitor. By decreasing the sizes of the electrical devices that constitute, for example, a memory cell and the sizes of the conducting lines that access the memory cells, the memory devices can be made smaller. Additionally, storage capacities can be increased by fitting more memory cells on a given area in the memory devices.
The continual reduction in feature size places ever greater demands on the techniques used to form the features. Photolithography, for example, is commonly used to pattern features, such as conductive lines. When dealing with photolithography, the concept of pitch can be used to describe the sizes of these features. Pitch is defined as the distance between an identical point in two neighboring features. These features are typically defined by the spaces between themselves, spaces that are typically filled by a material, such as an insulator. As a result, pitch can be viewed as the sum of the width of a feature and of the width of the space on one side of the feature separating that feature from a neighboring feature. However, due to factors such as the optics, or the wavelength of light used in the technique, photolithographic techniques each have a minimum pitch below which a particular photolithographic technique cannot reliably form features. Thus, the minimum pitch of a photolithographic technique is an obstacle to continued feature size reduction.
Pitch doubling, pitch multiplication, or spacer defined double/quadruple patterning is a method for extending the capabilities of photolithographic techniques beyond their minimum pitch. During a pitch doubling process a spacer film layer is formed or deposited over an existing mask feature. The spacer film is then etched, preferably using a directional etch, such as a reactive ion etch, thereby leaving only the spacer, or the material extending or formed from the sidewalls of the original mask feature. Upon removal of the original mask feature, only the spacer remains on the substrate. Thus, where a given pitch previously included a pattern defining one feature and one space, the same width now includes two features and two spaces, with the spaces defined by, in this instance, the spacers. As a result, the smallest feature size possible with a photolithographic technique is effectively decreased. While the pitch is actually halved in the above example, this reduction in pitch is referred to as pitch “doubling,” because the linear density of features has been doubled. The process as described above can be performed additional times, using the newly formed spacers as the original mask feature to again reduce the pitch by half, or quadruple the linear density of features.
In spacer applications, such as in the pitch multiplication process as described above, materials with specific etch characteristics are required for device fabrication. The original mask feature in a pitch multiplication process is typically thermal silicon dioxide (SiO2), with hydrofluoric acid used to etch, or remove, the mask feature. It is preferable that the thermal SiO2 mask feature is etched away completely while the spacer material remains intact. Therefore, spacer film materials with lower etch rates than that of thermal SiO2, for example in hydrofluoric acid, are needed.
Additionally, because spacers are formed on the sidewalls of pre-patterned features, spacer films are preferably conformal. In order to achieve this conformality, deposition techniques such as atomic layer deposition (ALD) or plasma enhanced atomic layer deposition (PEALD) are typically used. The template materials used in pitch multiplication processes can also lower the allowed thermal budget, thereby favoring lower temperature deposition techniques such as PEALD.
Titanium dioxide (TiO2) is a material which has favorable etch selectivity compared to thermal SiO2, and can be conformally grown by PEALD at relatively low temperatures. Tetrakis(dimethylamido)titanium (TDMAT) or other alkylamides are typically used as titanium precursors, while O2 plasma is typically used as an oxygen precursor. TiO2, however, crystalizes easily, for example because of extra energy like plasma or temperature, causing roughness in the spacer film which renders the resulting spacer unacceptable for its intended use. Although smooth, amorphous, TiO2 possibly could be grown as a thin film up to 20 nm thick in some cases, the growth of thicker TiO2 films or the use of extra energy leads to crystallization.
Accordingly, there is a need for methods of forming or depositing conformal thin films that are smooth and amorphous, while retaining favorable etch selectivity.